This invention relates to waste water sludge treatment, and more particularly to a method and apparatus for mechanically shearing and breaking apart solid particulates in waste water sludge, prior to the dewatering treatment. The invention is applicable to waste water sludges (sludges) generated in a range of waste water treatments, e.g., municipal water systems, paper manufacturing plants, plating facilities, automobile plants, textile plants, and plastics manufacturing plants.
Commonly, waste water is treated to remove contaminants that would otherwise prevent reuse of the water in potable water systems or that would contaminate the ground water. Similarly, other waste liquids are treated to remove contaminants that might prevent reuse of the liquids in industrial processes. Such contaminants can include solid particulate matter, or dissolved acids, metallic salts and organic alcohols, as well as dissolved odorous gases. In the case of solid particulate contaminants, the contaminants are separated out of the liquid as a sludge.
The present invention is concerned with treatment of the waste water sludge after its generation and separation from the waste liquid. The invention is applicable to sludges generated in various types of waste liquids, i.e., water or other liquids. For convenience, the term "waste water" is used to generically describe waste liquids containing solid sludge contaminants.
Treatment of waste waters often involves pumping the waste water into a large settling tank. The water containing suspended solids is then allowed to remain in the settling tank, for a period of time sufficient to permit the solid particulates to gravitationally settle to the bottom of the tank as a sludge.
In another type of treatment, air bubbles are injected into the waste water prior to its admission to the tank; the air bubbles attach to the solid particulates to provide a lighter particulate condition. In the settling tank, the lighter particulates collect on the liquid surface, where they are skimmed off by an overhead traveling belt-type collector as a thickened sludge. The heavier particulates settle to the bottom of the tank as a second separate sludge.
Sludges can also be produced as a by product of an aerobic aeration process, wherein an aeration unit is included in an aeration tank to bring gaseous air (oxygen) into contact with the particulates. The air is consumed by microorganisms in the waste water to produce a microbial floc, i.e., activated sludge, and the floc settles to the bottom of the settling tank, where it can be removed from the effluent liquid phase.
Another process involves anaerobic digestion, wherein the waste water is placed in a closed tank. The closed system allows biochemical processes to take place, whereby some of the acids in the waste water are converted to methane gas and carbon dioxide, and solid particulates are thus consumed, or changed, to a liquid or gaseous form. A quantity of sludge remains as a by-product of the process. Typically, the tank is heated to an elevated temperature, at, or near, 95.degree. F., in order to keep the process ongoing. The methane gas is removed through a vent in the top wall of the tank, and the sludge is removed through a valved port in the tank bottom wall.
Sludge generated in the above-described conventional waste water treatments, is commonly disposed of by incineration, landfill operations, or ocean deposition. In some cases, the sludge is converted into fertilizer for agricultural use. A common problem associated with economical disposal of the various waste water sludges, is the high water content of the sludge. Typically the sludge will have a water content of from about ninety percent to about ninety five percent, with only five to ten percent of the sludge mass being solids. This fact is an obstacle to the economical disposal of the sludge, since it increases the total sludge quantity that has to be handled. The high water content also makes the sludge somewhat more difficult to handle, since the sludge is in a thick, viscous, semi-liquid condition.
In an effort to increase the solids content in the sludge, the raw separated sludge has been subjected to various dewatering treatments, including vacuum filtration, centrifugation, pressure filtration, and drying on sand beds. However, these post-formation dewatering treatments have only been partially successful. The sludges resulting from these dewatering treatments still have only relatively small solids contents. Typically, the solids content of so-called dewatered sludge is only about twenty to forty percent, with the remaining eighty percent to sixty percent, being water.
The high water content of so-called dewatered sludge is due to the fact that much of the water is contained within the pores of the solid particles or within narrow channels, formed between agglomerated particles. Each particle is a relatively porous cellular structure, that acts somewhat like a sponge, to absorb and retain water molecules within the particle mass. Conventional dewatering processes are relatively ineffective in drawing, or extracting, water from the pores formed within the solid particles.